IPT technology is an area of increasing development and IPT systems are now utilised in a range of applications and with various configurations. One such application is the use of IPT systems in so called ‘charging mats’ or pads. Such charging mats will normally provide a planar charging surface onto which portable electronic devices (such as smartphones) may be placed to be charged or powered wirelessly.
Typically, the charging mat will include a transmitter having one or more power transmission coils arranged parallel to the planar charging surface of the charging mat. The transmitter drives the transmitting coils so that the transmitting coils generate a time-varying magnetic field in the immediate vicinity of the planar surface. When portable electronic devices are placed on or near the planar surface, the time-varying magnetic field will induce an alternating current in the receiving coil of a suitable receiver associated with the device (for example a receiver incorporated into the device itself). The received power may then be used to charge a battery, or power the device or some other load.
A problem associated with charging mat design is ensuring that the inductive power transfer is adequately efficient. One approach is to require precise alignment between the transmitting coil and the receiving coil. This may be achieved, for example, by having markings or indentations on the planar charging surface so that when a user places the device on the charging mat alignment between the coils can be guaranteed. However, this approach is not ideal since it requires the user to place their device carefully onto the charging mat.
Another problem associated with charging mat design is enabling multiple devices to be charged simultaneously in an efficient and cost effective manner. Some conventional designs use a single large transmitting coil corresponding to the entire surface of the charging mat. In this instance, one or more devices may be placed anywhere on the surface of the charging mat. This allows more freedom in terms of where a user may place a device on the charging mat. However, the magnetic field produced by a large transmitting coil may not be uniform, with ‘weak spots’ towards the centre of the charging mat. Further, since the entire surface is being ‘powered’ it is possible that any portions of the surface not covered by a device being charged may be a safety hazard.
Another conventional approach for multi-device charging is to have an array of smaller transmitting coils. In order to provide efficient and safe power transfer, the charging mat detects the position of the devices using a suitable detection mechanism and activates the most proximate transmitting coil or coils. Though this allows more freedom in terms of where a user may place a device, like the single coil design, the boundary between adjacent transmitting coils can result in weak spots due to the cancelling effects of adjacent coils whereby receivers do not receiver sufficient power.
A further problem arises when a non-receiver is brought into the range of the transmitter, and an unwanted current (and therefore heat) is induced therein. These non-receivers are typically known as parasitic loads or foreign objects. Detection of the presence of a receiver device is conventionally possible, but it may also be necessary to identify the receiver as being compatible with the particular transmitter. Attempting to transfer power to non-compatible receivers may result in inefficient power transfer (thus, undesired energy loss), or transmitter and/or receiver failure.
An obvious solution to the problems outlined above is to include a manually operated power switch with the transmitter. Though this provides a means for controlling when the transmitter should be powered, it undermines the convenience that is a goal of many IPT systems. It also requires a user to manually switch off the transmitter when the receiver is removed and does not accommodate any parasitic loads that may be introduced into the vicinity of the transmitter without the user's knowledge.
The invention provides an inductive power transfer system and methods that achieve reliable and efficient wireless power transfer for multi-device powering or at least provides the public with a useful choice.